1. Field of the Invention
The present invention relates to an ink jet recording apparatus for gradational printing such that a plurality of ink drops are continuously discharged through nozzles.
2. Description of the Related Art
Conventionally known is an ink jet recording apparatus in which an actuator composed of an electromechanical transducer such as a piezoelectric element is operated by means of driving signals to increase or reduce the capacity of a pressure chamber that is stored with ink, whereby the ink is discharge through nozzles to print a pixel by gradation. Ink jet recording apparatuses of this type are described in Jpn. Pat. Appln. KOKAI Publication No. 4-250045 and U.S. Pat. No. 4,513,299, for example.
In the ink jet recording apparatus described in Jpn. Pat. Appln. KOKAI Publication No. 4-250045, the voltage or pulse width of driving signals is changed to vary the volume of each ink drop that is discharged through a nozzle, whereby the dot size of each ink drop that is dashed against a recording medium can be changed for gradational printing.
In the ink jet recording apparatus described in U.S. Pat. No. 4,513,299, the number of driving pulses is controlled to discharge a plurality of ink droplets through nozzles and change the number of droplets to be discharged, whereby the dot size of each ink drop that is dashed against a recording medium can be changed for gradational printing.
In the case of the former gradational printing, it is hard considerably to change the volume of each discharged ink drop. Therefore, the latter gradational printing is superior to the former one in changing the dot size at a high rate.
In the latter gradational printing, compared with the former one in which the volume of one discharged ink drop is controlled to form one pixel, however, a plurality of ink droplets must be discharged at a higher driving frequency. In order to prevent lowering of the speed of the latter gradational printing, therefore, the droplets must be discharged by means of driving pulses with a considerably high frequency.
If these driving pulses are continuously applied to the actuator, vibration of meniscuses in the nozzles that are generated by means of driving pulses for discharging directly preceding ink droplets is followed by vibration of meniscuses that are generated by means of driving pulses for discharging subsequent droplets. Accordingly, the vibration of the meniscuses becomes so intensive and disturbing that ink in the nozzles involves air bubbles. If the ink in the nozzles thus involves air bubbles, the speed of discharge of ink drops lowers, and in some cases, no ink drops can be discharged.